The rubber effect in CU-Zn-Al martensite

Abstract

Rubber like behavior has been observed in several noble metal alloy martensite , as In-Tl (1,2), A u - C d ( 3 , 4 , 5 ) Cu - A u - Z n ( 6 ) , C u - A l - N i ( 7 ) and C u - A l - Z n ( 8 ) . Crystals of these alloys when deformed plastically in the martensitic State regain their original shape after the stresses are released. Only if the sample remains too long in the stressed state can the rubber effect disappear with the martensite remaining deformed after unloading. This behavior indicates that thermally activated time dependent processes are important. In most of the alloys mentioned the martensite is twinned and stresses lead to the displacement of the twin interfaces, which return to their original position after unloading. In the Cu - Zn - Al alloy (8) the rubber effect is observed in untwined single crystalline martensite. Only during stressing new plates appear with in the original martensite. On releasing the stresses these newly formed plates disappear again. The stressing - unloading cycle can be repeated without a change in the rubber - like behavior. Why do the interfaces go back to their original position s in the twinned martensite or disappear completely in the Cu - Zn - Al alloy? Two explanations have been offered: Birnbaum and Read (4) suggested that in the martensite order faults between split super - dislocations exist, which on t winning go over to high energy defects that change the volume free energy of the twinned martensite, thus providing a force that drives the interface back to its original position when the external stress disappears. Rubber like behavior can also be expected if twinning requires in addition to the macroscopic twin shear a reshuffling of the atoms to the correct position which is time dependent (5,6). Lieberman et al (5) used this model to explain the rubber effect in Au - Cd twinned martensite, which have the more complex Olander structure that is absent in most other martensite. The initial report of a rubber - effect in Cu - Zn - Al alloys (8) has left a great number of questions unanswered. In this paper therefore further experiments are described which serve to clarify the crystallography of the transformation. In a later paper a mode l will be presented which attempts to explain the rubber like behavior in these alloys.